Chrome-syntan pretannage followed by dry chrome tannage



United States Patent sen, Germany, a corporation of Germany No Drawing. Filed July 3, 1962, Ser. No. 207,418 Claims priority, application Girinany, July 19, 1961,

3 Claims. (51. 8-94.26)

The invention relates to and has as its object a new and economical rapid tanning process for the production of medium heavy and heavy leathers. According to this process the tanning of the hides, pret-anned in the usual manner, is finished in the drum with vegetable or synthetic organic tanning agents in powder form without liquor within a short time.

The production of medium heavy and heavy vegetable leathers has been based, up to now, chiefly on methods which can be summarized as counter current methods. The tanning is carried out by preparation in the lime house, the so-called raw hides, being first treated with weak spent tan liquors which are frequently utilized in the form of suspenders. The course of treatment consists of a series of pits; the hides are placed into the first pit and gradually pass through the remaining colour pits, the concentration of the tan liquor increasing from pit to pit. The final tanning takes place in strong liquors either in pits or in the drum. The addition of fresh extracts is effected at this last stage according to requirement.

Systems of the above type as well as rapid tanning processes of similar arrangement with liquors of higher con centrationare characterized in that, they require, inter alia, a large excess of tan liquors. Since the vegetable tannins age and are changed not only by oxidation and biological processes but also by impurities and by salts imported with the hide, uniform results with regard to the external appearance and the properties of the final product can only be obtained by very careful time consuming supervision and even then without attaining complete satisfaction. I

Since in this tanning system the fresh hides must be treated with partly spent liquors having a low concentration of tanning agents, the time required for producing leather in this way is considerable and varies from several weeks to several months.

Numerous attempts have, therefore, been made to replace the so-called counter current principle and attempts were made in the last century to shorten the tanning process by increasing the concentration of the tan liquors. No method of this kind for the production of heavy vegetable leathers could hitherto be widely introduced.

In order to understand the present process it is necessary to explain in detail the basic difficulties arising in the tanning of unsplit hides of large cattle with powdered tanning extracts. It has been ascertained that experience gained from previously known tanning methods cannot be utilized in the technique of tanning with powdered extracts. Thus, for example, the observation that the tanning agent penetrates more quickly into the raw hide if the concentration of the liquor is increased, is correct only within a certain range of concentration. It is not possible to tan a limed and delimed hide, prepared in the lime house in the usual way, by means of powdered extracts in the absence of liquor in 'the drum because the tanning agent removes water from the interior of the untanned layers of the raw hide. By the removal of water and the thus concurrent volume contraction of the raw hide to be tanned the passageways necessary for 3,253,879 Patented May 31, 1966 the penetration of the tanning agent are correspondingly narrowed, until finally the diffusion of the tannins is prevented completely. This phenomenon which is denoted as reversible case-hardening is of decisive importance in rapid tanning with concentrated tanning extracts, and especially for tanning with powdered tanning extracts without liquor.

A further example of the different-tanning mechanism in tanning with powdered extracts without liquor is the largely differing effect of the same tanning agent in the tanning with partly spent liquors of low and medium concentration on the one hand, and in tanning with powdered extracts on the other hand. Thus, for example, aqueous solutions of mimosa extracts which are employed in hitherto customary processes, are distinguished by a rapid tanning action, whereas chestnut extract solutions of the same concentration show, under otherwise identical conditions, result in a distinctly slower penetration into the hide. In contrast to this it has been found that in the tanning with powdered extracts, the

minosa exract :is substantially slower in absorption by advantage, by subjecting the raw hides to be tanned to.

a-pretanning by means of pretanning agents which are formed by mixing normal or basic tanning chromium salts with synthetic tanning agents which are prepared by known methods frorneither phenols by condensation with sulphites and/ or bisulphites and aldehydes, or from methylol compounds of phenols by condensation with sulphites and/or bisulphites, optionally in the presence of ketones or aldehydes, such as are obtained by methods well known to those skilled in the art. Such pretanning agents can be used alone or mixed with purified sulphite Waste liquors, known synthetic auxiliary tanning agents or synthetic exchange tanning agents. The pretanning takes place in an acid medium. This pretanning can be carried out in pits, in paddles, or in drums. The hide material, pre-treated in this manner, is uniformly tanned through without liquor, with powdered vegetable extracts while agitated in the drum for a short time.

Elaborate tests have shown that with other pretanning agents such as are employed for pretanning in the hitherto customary tanning methods, for example pretanning with formaldehyde, with synthetic 'auxil'iairy tanning agents, obtained by condensation of naphthalene sulphon-ic acid with formaldehyde, with sulphomethyl-ated phenol or cresol condensation products, withycondensation products from phenol, phenol sulphonic acids 'and/ or naphthalene sulplhonic acids with formaldehyde, no satisfactory results can be achieved regarding prevention of the undesirable phenomenon of oase-hardening-and formation of dressed grain.

The pretanning with the described. complex chromic taning agents is distinguished, inter alia, in that a small quantity of about 3% of these pretanning agents already effects a penetration of the cross section of the raw hide. By this pretreatment the hide material is not only rendered resistant to the dehydrating action of powdered extracts which,. during tanning of hides not pre-treated, gives rise to an interruption of the tanning process, dc. to case-hardening, but, at the same time, the grain is rendered resistant, thus counteracting the formation of the undesirable drawn grain. Moreover, the surface of the hides is preserved by the described pretanning agent, so that leathers with a very light and uniform leather colour result.

In contrast to a usual chromium pretanning with a pickle and tanning chromium salts, the leathers pretreated with the complex chromium tanning agents show only a small content of chromium oxide (about 0.3%) which in no way detrimenta lly influences the properties of sole leathers. It is well known that higher quantities of chromium oxide such as are present in customary vegetable leather pretanned with basic chromium salts, and salts imported by the pickle disadvantageously influence the properties of combined-tanned sole leathers prepared by these methods.

It has further been found that the tanning with powdered extracts without liquor can be considerably accelerated and the evenness of the distribution of the tanning agent improved by starting the final tanning with powdered vegetable tanning extracts, with a synthetic tanning agent in form of powder without liquor. Advantageously, well dispersing and readily soluble so-called exchange tanning agents are used for this purpose which possess a good self-tanning action, such as are obtained for example by heating in acid solution aromatic sulfonic acids with formaldehyde and unsulionated dihydroxy diphenyl sulfones. In place of the sulfonic acids use can also be made of its formaldehyde or sulfu-ryl chloride condensation products. Suitable aromatic sulfonic acids for this process are, for instance, mixtures obtained by sulfonating naphthalene in concentrated sulfuric acid upon heating at 140 to 160 C. or corresponding sulfonation products of tetra-hydronaphthalene (of. German patent specification 611,671).

It has also been found that the favourable effect of these preliminary tanning agents which are used for starting the powder tanning can be increased by adding quantities of about 5-10% of their own weight of watersoluble neutral salt, as well as of medium strong organic acids in a quantity of about l-% referred to the synthetic tanning agent.

Solid as well as liquid acids can be employed. The total duration of the tanning with the synthetic preliminary tanning agent and of the vegetable final tanning, even with a thick hide material, amounts to no more than between about .10 and 36 hours. The tanning through is, even in the densely structured parts of the hide, very uniform, which has been established by numerous layer analyses of the leathers prepared according to the new method.

The new process can be carried out with any of the known vegetable powdered or solid tanning extracts. It is true that there are gradual differences in the rate of tanning through between the particular extracts, but the new process permits in each case the tanning of even heavy hides. It has already been explained that the tanning mechanism of the new method differs in principle from the usual counter current processes. The well known selective absorption of the tanning agent does not take place whilst leaving behind the non-tanning agents. In contrast, in the new tanning-process, a uniform penetration of the hide by the whole water-soluble part of the vegetable extracts is brought about. At the end of the tanning there remains a small quantity of a residual liquor of comparatively low concentration.

Compared with rapid tanning processes with concentrated liquid extracts, the new process has the considerable advantage of no additional water being imported by the tanning agent into the tanning system, thus preventing the formation of a larger amount of residual liquor. 'I he considerably accelerated tanning brought about by the very high initial concentration of the tanning agent leads to a rapid resistance of the grain i ayer sensitive to drawn grain.

It is especially surprising that it is possible to carry out a tannage of this kind with powdered tanning agents on a material which has been rendered acidic by the pre-tanning. In spite of the acidity of the raw hide, smooth, grain-resistant, non-drawn leathers are obtained without showing the phenomenon of case hardening.

Besides many other methods attempting to accelerate the tanning by increasing the concentration of the tanning agent, a process has already been described in the US. patent specification 2,955,904 supposed not possessing the disadvantages brought about in the known rapid tanning methods. This method is characterized by the following '3 working processes:

(1) Pretanning with a non-vegetable tanning agent which is taken from the group of syntans, chromium tanning agents, quinones, formaldehyde, alum and sulphur tanning agents.

(2) Mechanical squeezing off the water of the pretanned hides by at least 30% of the original weight of the hides, but preferably by 50% (3) Final tanning with powdered vegetable extracts or aqueous solutions of theseextracts of at least 25 B.

It has however been found that this process is not applicable in practice when solid or powdered tanning agents are used, because with the working method described above the pretanned hides are either case-hardened, or the tanning comes to a standstill in the absence of a diffusion medium.

The following examples are given for the purpose of illustrating the invention.

Example 1 1000 kg. of unsplit cattle butt, after being treated in the lime house in the manner customary for sole leather, are placed in a pit which is filled with an aqueous solution of a complex chromium tanning agent of the following composition: 450 parts by weight of the condensation product prepared according to the disclosure given below are mixed with a solution of 300 parts by weight of chromium sulphate in 300 parts by weight of hot water and dried by a spraying process.

The concentration of the complex chromium tanning agent in the pretanning bath amounts to 12 kg. per cu. m. The raw hides remain in the pretanning bath for 24 hours and, during this time, the pretanning agent strikes through them. The hide material takes up about 3% of the pretanning agent, referred to the weight of the raw hides. After appropriate addition (34% referred to the weight of the hides) the pretanning bath is used for treating a new batch. Depending on deliming, a fresh bath can,

after appropriate addition, of pretanning agent and acid be used for a great number of batches.

The hide material thus pretreated is subjected to an intermediate treatment in the drum with 10% of a powder, neutral, synthetic tanning agent, obtained by condensation of 4,4-dihydroxy diphenyl sulphone and naphthalene sulphonic acid with formaldehyde, to which 10% of ammonium sulphate and 8% of glutaric acid, referred to the syntan proportion, are added, and subsequently finally tanned with 30% of powdered sulphited quebracho extract without liquor. The duration of the tanning in the drum amounts to 24 hours. The hides thus tanned are finished in the usual manner.

The condensation product referred to in the first paragraph of this example was obtained according to the teaching in Example 1, 3, 4 or 5 of German patent specification 1,053,517, i.e.:

(a) parts by weight of crude cresol and 50 parts by weight of phenol are condensed with 245 parts by volume of 30% formaldehyde in the presence of 112 parts by weight of 50% sodium hydroxide at about 70 C. for 3 hours. Sulfur dioxide is blown into the precondensate thus obtained untilneutral reaction of the mixture is reached. (pH 6-7). After an additional hour the product can be dissolved clear in dilute acids (Example 1 of German Patent 1,053,517).

(b) In accordance with the method given under (a) 200 parts by weight of crude cresol are condensed with 200 parts by volume of 37% formaldehyde in the presence of 52 parts by weight of solid sodium hydroxide at 85 C. within 3 hours. After sulfur dioxide has been blown into the precondensate formed, the sulfitation reaction is carried out at a temperature of 95 for one hour (Example 3).

(c) 300 parts by weight of phenol are mixed with 96 parts by weight of 45% soda-lye and 300 parts by volume of 30% formaldehyde added with stirring. The mixtureis refluxed for one and a half hours and then sulfur dioxide blown into the mixture until neutral reaction is reached. The condensation product is soluble in water. After 30 minutes further reaction it is acidified with acetic acid.

Instead of the above components 300 parts by weight of phenol can be condensed with 300 parts by volume of 30% formaldehyde and 280 parts by weight of 40% of sodium bisulfite at 98 C. (Example 4).

(d) 320 parts by weight of technical cresol (composed of about 20% phenol, 62% o-cresol, 5% m-cresol and 8% p-cresol) are mixed with 161 parts by weight of 45% soda-lye and condensed with 350 parts by volume of 30% formaldehyde. The condensation reaction is carried out at 98% C. within 35 minutes and subsequently sulfur dioxide blown into the mixture until the reaction is weakly acid. After further 90 minutes reaction time a clearly soluble product is obtained which is acidified with acetic acid (Example 5).

Example 2 500 kg. of split sole leather are tumbled in the drum for 3 hours with 500 kg. of water and 5% of the complex 7 chromium tanning agent described in Example 1. After this pretanning the residual liquor is drawn off and 100 kg. each of powdered sulphited quebracho extract, mimosa extract and chestnut extract placed into the drum. After a tumbling period of hours the split sole leather is saturated and evenly tanned through. The thus tanned split sole leathers are finished in the usual way. A small quantity of a comparatively thin residual liquor remains in the drum which is rejected.

Example 3 6 mixture of powdered sulphited quebracho extract and mimosa extract. The total duration of tanning in the drum amounts to 6 hours. The split cow hides tanned according to this process are fat-liquored in the customary way and finished.

We claim:

1. Process for the rapid tanning of medium and heavy leathers consisting essentially of (A) pretanning delimed hides with a normal or basic tanning chromium salt and a syntan obtained by the condensation of a phenol with an aliphatic aldehyde and with a member selected from the group consisting of a sulfite and a bisulfite, and (B) finally tanning by drumming the hide with an effective amount of a powdered vegetable tanning extract only.

2. Process for the rapid tanning of medium and heavy leathers consisting essentially of (A) pretanning delimed hides with a normal or basic tanning chromium salt and a syntan obtained by the condensation of a phenol with an aliphatic aldehyde and with a member selected from the group consisting of a sulfite and a bisulfite,

(B) drumming the pretanned hides before final tanning with a powdered well dispersing and readily soluble'exchange tanning agent; and

(C) finally tanning the hides by drumming with an effective amount of a powdered vegetable tanning extract only.

3. The process of claim 2 wherein the said exchange tanning agent is applied in combination with a member selected from the class consisting of about 5 to 20% of an inorganic neutral water-soluble salt and about 1 to 10% of medium to strong organic acid, the percentage being referred to the weight of the said exchange tanning agent.

References Cited by the Examiner UNITED STATES PATENTS 2,955,904 10/1960 Schnoller 894.21 2,997,364 8/ 1961 Komarek et a1. 8--94.26 3,010,779 11/1961 Komarek et a1. 8--94.27

FOREIGN PATENTS 118,155 3/1944 Australia.

611,671 4/1935 Germany.

858,561 12/1952 Germany. 1,053,517 12/1960 Germany.

310,823 5/ 1929 Great Britain.

OTHER REFERENCES Chen: Syntans and Newer Methods of Tanning, pages 16 and 16, pub. by the Chemical Elements, South Lancaster, Mass., 1950.

NORMAN G. TORCHIN, Primary Examiner.

ABRAHAM H. WINKELSTEIN, Examiner.

v D. LEVY, Assistant Examiner. 

1. PROCESS FOR THE RAPID TANNING OF MEDIUM AND HEAVY LEATHERS CONSISTING ESSENTIALLY OF (A) PRETANNING DELIMED HIDES WITH A NORAML OR BASIC TANNING CHROMIUM SALT AND A SYNTAN OBTAINED BY THE CONDENSATION OF A PHENOL WITH AN ALIPHATIC ALDEHYDE AND WITH A MEMBER SELECTED FROM THE GROUP CONSISTING OF A SULFITE AND A BISULFITE, AND (B) FINALLY TANNING BY DRUMMING THE HIDE WITH AN EFFECTIVE AMOUNT OF A POWDERED VEGETABLE TANNING EXTRACT ONLY. 